This invention relates generally to a system and method for sensing the passage of a member past a predetermined location along a tubing disposed in an oil or gas well. More particularly, but not by way of limitation, the present invention relates to a system and method for assisting the proper placement of a cement slurry in an oil or gas well.
Various objects may need to be dropped or pumped into an oil or gas well during its creation and completion. For example, when a casing or liner is installed in a well borehole, two cementing plugs may be released on either end of (one in front of and one after) a cement slurry that is pumped through a tubing (which can include the casing or liner itself) into the well. The first, lower cementing plug separates the cementing slurry from the drilling mud or other fluid already in the well, and this first plug drops into the lower part of the well when it reaches the lower end of the tubing (more specifically, it typically lands on a float collar). The second, upper cementing plug separates the cement slurry from a spacer or other following fluid pumped behind the cement slurry to push it around the lower end of the tubing and up the annulus between the casing or liner and an outer tubular or the wall of the borehole. These cementing plugs are typically made of a relatively soft material so that they can be readily drilled out by a conventional drill bit as the depth of the well is increased after the casing or liner has been set.
It is important to know whether the cementing plugs have properly released into the flow stream because if they have not, unwanted mixing between the cement slurry and other fluids can occur, and improper placement of the cement slurry in the well and improper bonding of the casing or liner can result. This need to detect proper release of cementing plugs has been known and attempts to satisfy it have been proposed or made.
Although various types of detectors for detecting the passage of objects, such as cementing plugs, in tubing disposed in oil or gas wells have been disclosed, these types are not necessarily reliably sensitive to the particular object that is to be monitored. For example, a mechanical type of detector may become fouled (such as by becoming cemented) and non-functional in the harsh oil or gas well environment where it is used. As an example of another shortcoming, a type of detector that includes a metallic member mounted on the object may create a drill-out problem if the metallic member is made of a material that cannot be readily drilled by conventional drill bits used in oil or gas wells. In view of at least these shortcomings, there is the need for an improved detector system and method which clearly indicates that the particular object to be monitored has been detected and which does not impede subsequent drill-out.